This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-373067, Dec. 28, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a semiconductor laser device, particularly, to a semiconductor laser device provided with a light absorption film having an aperture on the outside of a light-emitting surface.
In order to improve the recording density of an optical disc, required are a light source and an optical system capable of converging the laser light on a minimal spot. In general, the diffraction-limited spot diameter s of the converged light relative to the wavelength xcex of the light source and the numerical aperture NA of the converging lens is determined by formula (1) given below:
i.e., s=cxcex/NAxe2x80x83xe2x80x83(1) 
When it comes to a laser light having a cross sectional light intensity conforming with, for example, the Gaussian distribution and a diameter in which the light intensity in the edge portion is 1/e2 times as high as the light intensity in the central portion, the coefficient c in formula (1) is 0.67. In general, the numerical aperture NA of the lens is at most 1. It follows that it is impossible for the diffraction-limited spot diameter s to be smaller than cxcex, as apparent from formula (1).
As apparent from formula (1), an effective method for obtaining a spot light having a minimal diameter is to shorten the wavelength xcex of the light source. However, in the case of using a semiconductor laser, the shortening of the wavelength xcex is limited. Also, if the wavelength of the light source is shorter than that of an ultraviolet light, it is impossible to use the conventional optical system because of the restriction in the transparent region of the lens material.
On the other hand, as a method for exceeding the limit represented by formula (1), it is proposed to utilize a solid immersion lens (SIL) or an optical near field. The optical near field is generated when a laser light passes through a circular aperture mounted at the light-emitting edge and having a diameter smaller than the wavelength xcex of the light source. The optical near field thus generated is utilized by disposing a disc plane in the vicinity of the aperture. To be more specific, a laser light is formed into an optical near field having a diameter smaller than the diffraction-limited spot diameter s when the laser beam passes through the aperture, and the optical near field thus formed is utilized for recording information in an optical disc and for reading the recorded information from the optical disc.
However, a serious problem is inherent in the optical near field that the throughput efficiency of the optical near field is very low. Specifically, the aperture is formed in general in a plane of a light absorption material having a large optical absorption. It should be noted that a material having a very large absorption loss and a thickness large enough to inhibit the light transmission such as a metal material is used as the light-absorbing material so as to inhibit the light transmission in regions other than the aperture.
When passing through the aperture of the light-absorbing material, the laser light is absorbed by the light-absorbing material in the vicinity of the aperture, with the result that the laser light intensity is rendered insufficient on the emission side. In other words, the throughput efficiency of the optical near field is very low and, thus, it is impossible to use the optical near field for the optical recording/reading.
On the other hand, it is conceivable to use a high power laser as a measure for making up for the low throughput efficiency of the optical near field. However, in the construction that a light-absorbing material is mounted on the facet of a high power laser, the temperature in the vicinity of the facet is markedly elevated by the heat generation caused by the light absorption so as to deteriorate the laser facet. It follows that this measure is not practical.
As described above, in the conventional semiconductor laser device in which a small aperture is formed in the light-emitting surface, the efficiency for the laser light to pass through the aperture is very low, making it impossible to use the conventional semiconductor laser device for the optical recording. On the other hand, if a high power laser is used as a measure against the low throughput efficiency, the temperature in the vicinity of the laser facet is markedly elevated so as to deteriorate the high power laser.
An object of the present invention is to provide a semiconductor laser device capable of minimizing the spot diameter of the laser light and high in the optical throughput efficiency through an aperture.
The present invention has been achieved on the basis of the properties found by the present inventors that, when the aperture width is very small, the degree of light absorption differs depending on the polarizing direction of the laser light. It should be noted that the technical idea of the present invention resides in that the direction of the short aperture width of a small aperture is set parallel to the polarizing direction of the semiconductor laser element so as to improve the optical throughput efficiency through the aperture.
To be more specific, the present invention provides a semiconductor laser device comprising a semiconductor laser element and a light-absorbing film having an aperture formed on the outside of the light-emitting surface of the semiconductor laser element, characterized in that the aperture is formed such that the aperture width W1 in a direction parallel to the polarizing direction of the laser light is smaller than the aperture width W2 in a direction perpendicular to the polarizing direction.
The present invention also provides a semi-conductor laser device comprising a semiconductor laser element and a light-absorbing film having an aperture formed on the outside of the light-emitting surface of the semiconductor laser element, wherein the aperture is formed such that the aperture width W1 in a direction parallel to the polarizing direction of the laser light is smaller than half the oscillation wavelength of the semiconductor laser element, and the aperture width W2 in a direction perpendicular to the polarizing direction is larger than the aperture width W1.
The semiconductor laser devices according to preferred embodiments of the present invention are featured mainly as follows:
(a) A dielectric film is arranged between the light-emitting surface and the light-absorbing film. It is possible for the dielectric film to be formed, as desired, to fill the aperture.
(b) The aperture width of the aperture in a direction parallel to the polarizing direction of the laser light is set to fall within a range in which the absorption loss of the laser light is made smaller by at least one place than that in the case where an aperture of the same width is formed to extend in a direction perpendicular to the polarizing direction of the laser light.
(c) The width of the aperture in a direction parallel to the polarizing direction of the laser light is shorter than one-third of the oscillating wavelength of the semiconductor laser element.
(d) The semiconductor laser element is of an edge-emitting type and has an oscillation mode of TM mode.
(e) The light absorption film is made of a metal.
(f) An insulating film is arranged between the light-emitting surface and the light-absorbing film, and the optical thickness of the insulating film falls within a range of between 0.05xcex and 0.35xcex relative to the oscillating wavelength xcex.
The present inventors have found that the loss of the laser light in the small aperture is dependent on the polarizing direction of the laser light and on the shape of the aperture. To be more specific, the loss is increased if the aperture width in a direction perpendicular to the polarizing direction of the laser light is narrowed, and the loss is not increased even if the aperture width is narrowed in a direction parallel to the polarizing direction of the laser light. The specific reason for this principle will be described herein later.
It follows that, if the aperture width W1 in a direction parallel to the polarizing direction of the laser light, the aperture being formed on the outside of the light-emitting surface of the semiconductor laser element, is made smaller than the aperture width W2 in a direction perpendicular to the polarizing direction of the laser light as in the present invention, it is possible to obtain a spot light having a smaller diameter without increasing the absorption loss around the aperture. It follows that it is possible to minimize the spot diameter of the laser light so as to obtain a semiconductor laser device having a high light transmission efficiency through the aperture.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.